(a) Field of the Invention
The present invention relates to a cell search system for a mobile station in a time division duplex system, and a method for the same. More specifically, the present invention relates to a cell search system for a mobile station in a time division duplex system and a method for the same in which the mobile station searches secondary synchronization codes for initial synchronization acquisition without determining a threshold value in the system, and then enters the checking mode, thereby reducing the initial synchronization acquisition time.
(b) Description of the Related Art
In a time division duplex (TDD) system, the same frequency is used in transmitting and receiving data. The TDD system employs a time division multiple access method in which each 10 ms long frame is divided into 15 time slots to be shared among multiple users. The TDD system is one of the wireless access standards for IMT-2000 using a code division multiple access method to support multiple simultaneous users.
An advantage of the TDD system is that it is capable of adapting to determine the direction of data communications so as to provide asymmetric data services such as Internet efficiently, in the aspect of frequency.
FIG. 1 is an illustration of a sync channel configuration in a general TDD system.
In the TDD system, base stations send sync channels for each frame, as shown in FIG. 1, so that the mobile station performs an initial synchronization search.
The sync channel configuration shown in FIG. 1 is an example of the current standards in the TDD system. The sync channel is classified into primary synchronization codes (PSC) and secondary synchronization codes (SSC), and carries one PSC and three SSCs, each having a length of 256 on two slots per frame.
In FIG. 1, one frame consists of 15 slots having a length of 10 ms, and PP and PS are the code powers of the PSC and the SSC, respectively.
FIG. 2 shows an example of the code structure forming the sync channel of FIG. 1.
In the code structure of PSC CP and SSC CS as shown in FIG. 2, the position of the sync channel, if carried on two slots per frame and starting from slot k, is slots k, k+8, k+15, and k+23.
The SSCs are generated by multiplexing a 256-cycle Hadamard sequence by a 256-length code consisting of a combination of 8-length codes A={1, 1, 1, 1, 1, 1, −1, −1} and B={1, −1, 1, −1, 1, −1, −1, 1} and modulating {±1, ±j} every 256-length code.
The SSCs are generalized hierarchical Golay (GHG) codes, and they are constructed as a combination of A and B, as illustrated in FIG. 2.
Table 1 shows an example of SSCs modulated according to groups and slots. Three SSCs are modulated by {±1, ±j} and, as illustrated in FIG. 2, transmitted twice every frame, the same SSC being repeated every two frames.
TABLE 1CodeCodeFrame 1Frame 2GroupSetSlot kSlot k + 8Slot kSlot k + 8toffset01C1C2C5C1C3−C5−C1−C3C5−C1−C3−C5t011C−C3C5C1−C3−C5−C1C3C5−C1C3−C5t121jC1jC3C5jC1jC3−C5−jC1−jC3C5−jC1−jC3−C5t231jC1−jC3C5jC1−jC3−C5−jC1jC3C5−jC1jC3−C5t341jC1jC5C3jC1jC5−C3−jC1−jC5C3−jC1−jC5−C3t451jC1−jC5C3jC1−jC5−C3−jC1jC5C3−jC1jC5−C3t561jC3jC5C1jC3jC5−C1−jC3−jC5C1−jC3−jC5−C1t671jC3−jC5C1jC3−jC5−C1−jC3jC5C1−jC3jC5−C1t782C10C13C1C10C13−C14−C10−C13C14−C10C13−C14t892C10−C13C1C10−C13−C14−C10C13C14−C10C13−C14t9102jC10jC13C14jC10jC13−C14−jC10 −jC13C14−jC10 −jC13−C14t10112jC10−jC13C14jC10−jC13−C14−jC10 jC13C14−jC10 jC13−C14t11122jC10jC14C13jC10jC14−C13−jC10 −jC14C13−jC10 −jC14−C13t12132jC10−jC14C13jC10−jC14−C13−jC10 jC14C13−jC10 jC14C13t13142jC13jC14C10jC13jC14−C10−jC13−jC14C10−jC13−jC14−C10t14152jC13−jC14C10jC13−jC14−C10−jC13jC14C10−jC13jC14−C10t15163C0C6C12C0C6−C12−C0−C6C12−C0−C6−C12t16233jC6−jC12C0jC6−jC12−C0−jC6jC12C0−jC6jC12−C0t20244C4C8C15C4C8−C15−C4−C8C15−C4−C8−C15t24314jC8−jC15C4jC8−jC15−C4−jC8jC15C4−jC8jC15−C4t31
FIG. 3 is a flow chart showing a cell search method for a mobile station in a TDD system according to an example of the prior art.
As shown in FIG. 3, upon receiving a sync channel, in step 1, the mobile station searches PSCs with a matcher to detect a maximum value of PSCs, in step 2.
The mobile station decides in step 3 whether the maximum value exceeds a first threshold predetermined in the system. When the maximum value does not exceed the first threshold, the mobile station searches PSCs again; otherwise, when the maximum value exceeds the first threshold, the mobile station determines the position of the maximum value as that of the sync channel, in step 4.
Using the position of the sync channel and SSCs, the mobile station searches information, such as the group of the base station to which it belongs, slot boundary, and the position of the sync channel in the frame, in step 5.
The mobile station uses the information about the group to which it belongs to get the knowledge of four predetermined midamble codes and scramble codes, in step 6. Then the mobile station selects a maximum value of the four midamble codes and decides in step 7 whether the maximum midamble code exceeds a second threshold predetermined in the system.
When the maximum midamble code exceeds the second threshold, the mobile station decides that it has acquired its base station, and detects the scramble codes from the selected midamble code to demodulate broadcasting channels from the scramble codes, in step 8.
When the maximum midamble code does not exceed the second threshold, the mobile station decides that it has failed to acquire its base station, and searches PSCs again, in step 9.
The base station search method for the mobile station as described above has a problem in that it is difficult to accurately determine the threshold values for the PSC search, because the power of the received signal dramatically changes due to channel environments when the mobile station receives the signals from the base station.
This difficulty in determining the accurate threshold values for the PSC search causes an increase in the probability of false alarms or defectives according to the threshold values for the PSC search, and increases the cell search time.